Apparatus and method for separating fluids through a membrane

ABSTRACT

Apparatus for contacting first and second fluids at elevated pressures with a membrane such that one or more components of one of the fluids passes through the membrane into the other fluid, comprising a housing e.g. a pressure vessel ( 4 ), and a membrane module ( 20 ) housed in the pressure vessel and having a first fluid inlet ( 8   a ) and a first fluid outlet ( 8   b ) and a second fluid inlet ( 9   a ) and a second fluid outlet ( 9   b ), wherein a seal ( 61 ) extends round the membrane module and seals between an outside wall of the membrane module and an inside wall of the pressure vessel so as to separate the first fluid inlet from the first fluid outlet.

This application is a continuation-in-part of U.S. Provisionalapplication No. 60/187,051, filed Mar. 6, 2000.

The invention relates to apparatus and methods for separating one ormore components from a fluid through a membrane, at ambient or elevatedpressures, and to modules containing such a membrane. The basicprinciples of one type of separating process to which the presentinvention is applicable is described in WO 98/04339.

In modern industry, mass and heat transfer processes are among the mostcommon processes. Mass transfer processes include contacting columns andextraction processes. In a contacting column such as an absorber or adesorber, a gas is contacted with a liquid over a high surface area,allowing the desired chemical or physical mass transfer processes totake place in a controlled manner. Afterwards the gas and the liquid areseparately and usually continuously transported away from the column.Due to the intimate contact between the fluids, a parallel heat transferalso takes place in cases where the fluids have different temperatures.

Contacting devices can also simply be mixing devices with subsequentseparators, which is used in for example processes involving twoimmiscible liquids. These processes include liquid/liquid extraction.

Conventional equipment does however have certain limitations anddrawbacks. These include the limited gas to liquid ratios in columns,requirements as to orientation and in most cases also the fact thatentrainment of one of the fluids in to the other often takes place,causing nuisance in downstream equipment and/or loss of product. Forliquid/liquid extraction processes, the degree of immiscibility iscrucial.

Membrane contactors provide means of carrying out the processesmentioned above without these disadvantages. In a membrane contactor,two (or more) fluids are exposed to each other through a membrane. Themembrane has the property to restrain one or more components of eachfluid, while allowing a relatively higher proportion of selectedcomponents to pass through the membrane. The membrane may also insteadof being selective in itself only provide such surface properties thatfor example components in liquid form are not able to pass, butcomponents in gaseous form can. An example of such a membrane is a(human) lung, where blood is not able to pass through the lung“membrane”, but oxygen, water and CO₂ are readily transported throughdriven by the partial pressure difference of these on the two sides ofthe lung.

For industrial applications the membrane provides a physical barrierbetween the two fluids, thus eliminating problems related to mixing ofthe fluids. The transport of a fluid is constrained to either side ofthe membrane, thus allowing full flexibility in terms of turndown (fromzero to full flow) without affecting the other side of the membrane.This also allows the orientation of the unit to be arbitrary andindependent of gravity.

The membrane used in each process is selected to have the propertiesthat are best suited for that particular application.

In high pressure processes, the membrane contactor has to be housed in apressure vessel designed for the purpose. The requirements tofunctionality, strength and corrosion resistance at elevated pressureand temperature have to be met. A reliable and suitable means of doingthis is therefore required.

It is known from U.S. Pat. No. 5,916,647 and EP 941758 to providepressure vessels made of polyolefin and containing a bundle of hollowfibre membrane contactors. The bundle has a hollow centre via whichshell side fluid flows into and out of the bundle, and the hollow fibreends are potted to provide for tube side fluid flow into and out of thebundle. The pressure vessel is provided at each end with inlet andoutlet pipes disposed centrally so as to mate in the hollow centre ofthe bundle in a manner sealed from the tube side outlet and inlet. Theassembly of the bundle into the pressure vessel is therefore not simple.Moreover, the need for a hollow centre in this type of membraneconfiguration reduces the amount of active membrane area per unitvolume.

From U.S. Pat. No. 5,264,171 it is known to provide apparatus comprisinga housing which receives a bundle of hollow fibre membrane contactorswith a type of inlet/outlet configuration different from that describedabove. FIG. 1 of U.S. Pat. No. 5,264,171 shows a housing with a fluidinlet port and a fluid outlet port at opposite axial ends for tube sideflow, and a fluid inlet port and a fluid outlet port for shell side flowwhich are disposed axially inwardly of the axially opposite ends at thesides of the housing. An impermeable wrapping means covers the exteriorcylindrical surface of the bundle of hollow fibre membrane contactors.An opening is provided in the impermeable wrapping means to communicatewith the side inlet port of the housing, and a corresponding opening isprovided to communicate with the side outlet port of the housing. Theedges of the impermeable wrapping means surrounding the openings aresealed to the housing side inlet and outlet ports respectively, toprevent leakage from the shell side space.

It is therefore necessary in this arrangement to fit the bundle into thehousing in such a way that the openings in the impermeable wrappingmeans register with the housing side inlet and outlet ports, with theedges of the openings properly sealing with the ports. This tends tomake the assembly of the apparatus difficult, with a risk of unreliablesealing of the shell side space in the region of the housing side inletand outlet ports, for example if exact rotational alignment about thelongitudinal axis between the bundle and the housing is not achieved.

Viewed from a first aspect the invention provides apparatus forcontacting first and second fluids at elevated pressures with a membranesuch that one or more components of one of the fluids passes through themembrane into the other fluid, comprising a housing, and a membranemodule housed in the housing and having a first fluid inlet and a firstfluid outlet and a second fluid inlet and a second fluid outlet, whereina seal extends round the membrane module and seals between an outsidewall of the membrane module and an inside wall of the housing so as toseparate the first fluid inlet from the first fluid outlet.

With such an arrangement, the assembly of the membrane module into thehousing can be relatively straightforward, in that exact registration ofthe first fluid inlet and outlet with respective inlet and outlet portsof the housing is not critical. This may also enable the membrane moduleto be used in a conventional housing. For example, the membrane modulemay be used in a conventional pressure vessel without having tospecially design the pressure vessel. The membrane module may beretrofitted to an existing housing, e.g. pressure vessel.

Various types of membrane may be used in the membrane module, forexample paste extruded membrane tubes or spirally wound membranes.Preferably, a plurality of membrane tubes are provided. In a preferredembodiment, the membrane module comprises a plurality of membranelayers, each membrane layer comprising a plurality of membrane tubesarranged side by side with connecting portions connecting laterallyadjacent membrane tubes, and the membrane layers being stacked inalternation with spacers. Such an arrangement can achieve a veryeffective mass transfer across the membranes. The membrane layers arepreferably as described in U.S. Pat. No. 6,010,560.

It will be appreciated that the word “tube” used herein in relation tothe membrane is not intended to be limiting as to the diameter size ofthe tube and should therefore be understood to cover various diametersof tube, including tubes in the form of fibres.

The seal extending round the membrane module is preferably positioned atan intermediate location along the membrane module, spaced from axialends thereof.

Preferably, a pair of second seals each extend round the membrane moduleand seal between the outside wall of the membrane module and the insidewall of the housing, the second seals being axially spaced from thefirst-mentioned seal and on opposite axial sides thereof. Such secondseals can in effect define isolated inlet and outlet regions, namely aninlet region between the first seal and one of the second seals and anoutlet region between the first seal and the other of the second seals.

Appropriate sealing arrangements may be provided for the second fluidinlet and outlet. It is however preferred and advantageous for thesecond seals additionally to serve to separate the first fluid inlet oroutlet from the second fluid inlet or outlet.

The housing may be a pressure vessel or pipe spool or other form ofhousing. In a preferred embodiment, the housing has respective inlet andoutlet ports cooperating with the fluid inlets and outlets of themembrane module. In a particularly preferred arrangement: a first fluidinlet port of the housing is arranged to supply the first fluid to thefirst fluid inlet, which is provided in the outside wall of the membranemodule; a first fluid outlet port of the housing is arranged to receivethe first fluid from the first fluid outlet, which is provided in theoutside wall of the membrane module; a second fluid port of the housingis arranged to supply the second fluid to the second fluid inlet of themembrane module, which is provided at an axial end of the membranemodule; and a second fluid outlet port of the housing is arranged toreceive the second fluid from the second fluid outlet of the membranemodule, which is provided at the opposite axial end of the membranemodule. Preferably, the first inlet and outlet ports for the first fluidare disposed axially inwardly of opposite axial ends of the housing,preferably being laterally directed, and the second inlet and outletports for the second fluid are provided at the axially opposite ends ofthe housing, preferably being axially directed.

The housing may have more than one first fluid inlet port and more thanone first fluid outlet port. For example, the housing may have a pair ofdiametrically opposed first fluid inlet ports and a pair ofdiametrically opposed first fluid outlet ports.

In the case where a plurality of membrane tubes are used, it ispreferred for the first fluid to flow on the shell side of the tubes andfor the second fluid to flow on the tube side. Thus, in the preferredarrangement discussed above, the first inlet and outlet ports of thehousing, and the corresponding first inlet and first outlet of themembrane module, are provided for shell side flow; whilst the secondinlet and outlet ports of the housing, and the corresponding secondinlet and second outlet of the membrane module, are provided for tubeside flow.

It will be appreciated that by arranging a second seal axially betweenan end port and a lateral port of the housing, these ports may besealingly separated from each other. Thus, the first seal and the pairof second seals can advantageously isolate the four housing ports fromeach other by sealing between the outside wall of the membrane moduleand the inside wall of the housing. Four inlet and outlet regions caneffectively be defined at the point when the membrane module isinserted, e.g. axially, into the housing.

It is desirable to use the available space in the housing and thusoptimise the performance of the apparatus relative to its size.Preferably, therefore, the membrane module has a cross-sectional shapecorresponding substantially to that of the housing so as to fit closelytherein.

The membrane module is most conveniently assembled into the housing inthe axial direction. The housing may therefore have at least one fulldiameter flanged opening for insertion of the membrane module. In somecircumstances, such as for slender membrane modules, both ends may havea full diameter flange. Flanges do however add to the cost and weight ofthe housing and alternatively therefore an end cap may be welded to thehousing to close an end thereof, after insertion of the membrane module.If necessary, such a welded end cap can be burned off if the membranemodule needs to be changed after a period of service.

It is preferred for the first-mentioned seal to be placed in the housingbefore insertion of the membrane module into the housing. This isbecause the seal is ideally located and retained by a groove and ingeneral it is easier to provide a groove in the housing than in themembrane module. Preferably, therefore, the seal is received in a groovein the housing. It is particularly preferred to provide the groove in aseparate member which is secured to a main body of the housing, e.g. bywelding. This avoid having to form a groove directly in the main body,the inside of which, particularly in the case of a long slender housing,may be difficult to access.

The separate member may be secured inside a single main body of thehousing. However, if welding is used, the welding stresses may causedistortion and prevent proper sealing. This problem can be solved byproviding the housing with two body portions to each of which theseparate member is secured to form a connection between the bodyportions. The separate member may then be of greater external diameterthan the body portions thereby providing a strengthening flange which isnot significantly distorted during the securing process.

The second seals may also be placed in the housing before insertion ofthe membrane module. However, in the case of a long slender moduleproper axial alignment can be difficult to control. Hence, the secondseals may preferably be placed after insertion of the membrane moduleinto the housing, before closing off the housing with end caps, with orwithout flanges.

Plow of one of the fluids may be at an angle, such as a right angle, tothe membrane surface (this would be shell side flow in the case ofconventional membrane tubes or hollow fibres). Such crossflow iscommonly used in membrane separating processes. Preferably, however, afirst fluid flow path is defined along one surface of the membrane and asecond fluid flow path is defined along an opposite surface of themembrane, wherein the direction of flow of the first and second fluidsalong their respective paths is substantially parallel to said surfacesof the membrane. The first and second fluid flows are ideally indirections opposite to each other. These arrangements have been found towork particularly well in the preferred embodiments of the invention.

The membrane module preferably has at least one lateral opening in itsoutside wall to form the first fluid inlet and at least one lateralopening in its outside wall to form the first fluid outlet. It may bepreferred to arrange for the lateral opening(s) to be positioned on theside of the pressure vessel remote from the inlet/outlet port thereof,in order to obtain a more even flow into the membrane module and avoidexcessive flow velocities. In such circumstances, a degree of rotationalalignment will be necessary but it may not have to be exact.

Preferably, however an even flow into the membrane module withoutexcessive flow velocities is obtained by arranging the first fluid inletto permit flow of the first liquid laterally into the membrane modulesubstantially around its entire periphery. This arrangement isconsidered to be of independent patentable significance, as discussedfurther below.

In known systems, multiple membrane tubes are arranged in an axiallyextending bundle with their ends potted so that the fluid for tube sideflow can be directed into the axial end of the bundle and the fluid forshell side flow can be directed into the sides of the bundle inwardly ofthe axial ends. Efforts have been made in the art to optimize theefficiency of fluid to membrane contact but the known systems stillsuffer from a lack of use of the maximum available membrane surfacearea. In particular, shortcomings in the design of the inlet and outletarrangements for shell side flow often mean that there are membraneportions in dead spaces where little or no shell side flow takes place.For example, in the case of U.S. Pat. No. 5,264,171 mentioned above, theexternal housing of the membrane contactor apparatus has a single fluidinlet port which feeds into a single opening in the impermeable wrappingmeans in sealed registration with that port, with the result that themembranes on the side of the housing opposite the port may be in deadspace as far as shell side flow is concerned.

The known systems are therefore not as efficient as they could be, inthat not all of the membrane surface area is providing contact betweenthe tube and shell side fluid flows. We have invented an inletarrangement for a membrane bundle which improves the shell side flow inthe inlet region and therefore maximises the use of the availablemembrane surface area.

Viewed from another aspect, therefore, the invention provides membranecontactor apparatus comprising a bundle of axially extending membranetubes arranged for a first fluid to flow outside of said tubes and for asecond fluid to flow axially inside said tubes from an axial end of thebundle to an opposite axial end thereof, such that one or morecomponents of one of the fluids passes through walls of the membranetubes into the other fluid, the apparatus further comprising inlet meansdisposed axially inwardly of one of said axial ends of the bundle forintroducing said first fluid into the bundle, wherein the inlet meansextends peripherally round the bundle and is arranged to permit flow ofsaid first fluid laterally into the bundle substantially around itsentire periphery.

This arrangement can reduce dead spaces and maximise the axial length ofmembrane tubes available for fluid contacting. Moreover, it is generallydesirable for the bundle to be provided in a mechanically rigid form,particularly where it is to be inserted in a housing. Thus the bundlemay be provided as part of a membrane module, as discussed above, havingan outside wall. By arranging the first fluid inlet means to extendperipherally round the bundle, it is possible to avoid the use of asingle large opening in the outside wall which would result in anon-symmetrical load bearing capacity and potential adverse bendingduring installation of the bundle or even leakage past the seals causedby distortion.

The inlet means may comprise one continuous inlet opening in theperiphery of the bundle. However this may not always be practical from amechanical integrity point of view and there may be intervals betweenplural openings in the periphery of the bundle. Preferably, the inletmeans comprises a plurality of circumferentially spaced inlet openingsin the periphery of the bundle. The periphery of the bundle may bedefined by an impermeable outside wall around the bundle, such as theoutside wall of a membrane module.

In a preferred embodiment, the membrane tubes are arranged in aplurality of layers extending laterally across the bundle, for exampleof the type shown in U.S. Pat. No. 6,010,560. Flow within the bundle isgenerally easier parallel to the layers than passing from one layer toanother (although this is usually allowed for by suitable holes in thelayers and spacers if provided). Advantageously, therefore, the inletopenings are arranged in the periphery of the bundle so that eachmembrane layer is exposed to at least one of the inlet openings.

If the circumferential spacing between adjacent lateral openings in theoutside wall is too small, this can undesirably weaken the mechanicalintegrity of the wall. It is therefore advantageous to provide at leasttwo rows of circumferentially spaced inlet openings in the periphery ofthe bundle, with the centres of inlet openings in adjacent rows beingcircumferentially offset. With such an arrangement, every membrane layercan be-exposed to a respective inlet opening whilst an adequatecircumferential spacing between the openings in the same row is providedfor mechanical strength.

In the case of a membrane layer, it is possible to expose the layer to arespective inlet opening at only one side of the layer. This means thata membrane layer exposed on one side need not be exposed on the otherside, so that the circumferential spacing between adjacent inletopenings in the periphery of the bundle on that other side may begreater, allowing mechanical rigidity and strength to be retained. Thus,in this arrangement, some of the membrane layers are exposed to arespective inlet opening at only one side of the respective layer.

As discussed above in relation to the first aspect of the invention, thebundle may be housed in a housing having an inlet port for the firstfluid. In such an arrangement, one way of providing for flow laterallyinto the bundle substantially around its entire periphery is to providean inlet chamber arranged to receive the first fluid from the inlet portand extending round the periphery of the bundle. In the case of thepreferred bundle of circular cross-sectional shape, the inlet chamberwill be annular.

In order to avoid a high pressure drop and mechanical forces on thebundle, the first fluid flow speed should decrease from the inlet portto the periphery of the bundle. This can be achieved across the width ofthe inlet chamber, providing the width is greater than a certain size.Preferably, the width of the inlet chamber, measured between an insidewall of the housing and the periphery of the bundle, is greater than orequal to one quarter of the width of the inlet port.

It will be appreciated that the features described above in relation tothe inlet means of the bundle are generally applicable also toappropriate outlet means of the bundle. Thus the apparatus may compriseoutlet means disposed axially inwardly of the axial end of the bundleremote from the inlet means, the outlet means extending peripherallyround the bundle and being arranged to permit flow of the first fluidlaterally out of the bundle substantially around its entire periphery.The outlet means may comprise a plurality of circumferentially spacedoutlet openings in the periphery of the bundle, and/or an outlet chamberextending round the periphery of the bundle, the outlet openings and theoutlet chamber being analogous respectively to the inlet openings andthe inlet chamber.

The invention also extends to a membrane module for use in apparatus asdescribed herein and to mass transfer processes using such apparatus.

Certain preferred embodiments of the invention will now be described byway of example and with reference to the accompanying drawings, inwhich:

FIG. 1 is a side view of a membrane module;

FIG. 2 a is a cross-section on lines A—A of FIG. 1;

FIG. 2 b is a cross-section on lines B—B of FIG. 1;

FIG. 3 is a side view of a housing in the form of a pressure vessel forcontaining the membrane module;

FIG. 4 shows the pressure vessel in an open condition for insertion ofthe membrane module;

FIG. 5 is a side view of the pressure vessel showing the membrane modulecontained within;

FIG. 6 is a view, to an enlarged scale, of part of a modified housing.

The membrane module 20 comprises a bundle 1 of membrane tubes containedin a canister 2 made of a plastics composite material or metal. Thebundle 1 consists of several layers, each membrane layer comprising aplurality of membrane tubes 1 a arranged side by side with connectingportions 1 b connecting laterally adjacent membrane tubes, the membranelayers being stacked in alternation with spacers 1 c. The membranelayers are of the type shown in U.S. Pat. No. 6,010,560. As seen in FIG.2, the layers are arranged such that substantially the entire crosssection of the tubular canister 2 is occupied.

The membrane tubes are potted at the opposite ends of the canister bypotting 3. A thermosetting polymeric material is used as matrix materialin between the individual membrane tubes. In addition to ensuring thatinlet/outlet flow may only pass through the membrane tubes, i.e. tubeside flow, the potting ensures adhesion, load bearing and mechanicalstability to the membrane bundle. Axially inwardly of the potted ends,but adjacent thereto, the canister is provided with a set of openings 7a, 7 b for inlet and outlet flow on the membrane shell side. Each set ofopenings 7 a, 7 b comprises a pair of longitudinally spaced rows, theopenings in each row being circumferentially spaced from each adjacentopening. As shown, the centres of the openings in one row arecircumferentially offset from those in the other row. Thus, the openingsof the pair of rows are arranged so that every membrane layer is exposedto at least one opening.

Three annular seal surfaces 61, 62 and 63 are provided around theoutside of the canister 2. A first annular seal surface 61 is disposedat a position axially intermediate of the canister ends, the other twoannular seal surfaces 62, 63 being axially spaced from the first annularseal surface 61 towards the respective canister ends.

FIG. 3 shows a pressure vessel 4. At one end this has a full size flangeopening 5, enabling easy insertion of the membrane module 20 as shown inFIG. 4. In a modification, a flange opening 5 is provided at both endsand an additional flange opening 5 is therefore shown in dotted lines.At its axial ends the pressure vessel has an inlet port 8 a and anoutlet port 8 b. Axially inwardly of the ends, but adjacent thereto, thepressure vessel 4 has a sideways facing inlet port 9 a and a sidewaysfacing outlet port 9 b. In a further modification, a pair of inlet ports9 a and a pair of outlet ports 9 b are provided and these are shown indotted lines. Each port of a pair is diametrically opposite the otherport of the pair. The ports 8 a, 8 b are provided for tube side flow,whilst the ports 9 a, 9 b are provided for shell side flow.

A back pressure regulator 10 is provided downstream of the outlet port 8b and is arranged to receive a reference signal 11 indicative of thepressure at inlet port 9 a. The back pressure regulator 10 is arrangedto ensure that the differential pressure across the membranes does notexceed a predetermined amount, thereby ensuring that the membranes arenot damaged in use.

FIG. 4 shows three annular seals 61 a, 62 a and 63 a provided inside thepressure vessel for sealing engagement respectively with the threeannular seal surfaces 61, 62 and 63. At the flanged opening 5, eachflange has a bevelled edge so as to form a V-shaped groove when the endcap closes the pressure vessel 4. The seal 63 a is installed after themembrane module 20 has been inserted into the pressure vessel 4. Whenthe flanges are tightened together, the end seal 63 a is compressed.

Intermediate seal 61 a is supported in an annular groove formed in aseparate member in the form of a ring 30 welded inside the pressurevessel to the inside of its cylindrical wall 32. In the modificationshown in FIG. 6 the pressure vessel is formed as two body portions toeach of which a ring 30 incorporating a radial outer flange is securedto form a connection between the body portions. The ring is welded tothe body portions by outer welds 34 and inner welds 36. The ring 30shown in FIG. 6 also has an annular groove for supporting seal 61 a.

End seal 62 a is, like intermediate seal 61 a, supported in an annulargroove in a separate member where no flanged opening is provided at thatend. Both seals 61 a and 62 a are sufficiently flexible to give littleresistance during installation of the membrane module into the pressurevessel. If an additional flanged opening is provided at the end whereseal 62 a is located, then the arrangement will be the same as providedfor end seal 63 a.

FIG. 5 shows the membrane module 20 contained in the pressure vessel 4.It will be seen that once the membrane module is inserted in thepressure vessel, the annular seals 61 a, 62 a, 63 a ensure theseparation of the pressure vessel inlet and outlet ports as required.The three annular seals-effectively create four isolated inlet andoutlet regions within the pressure vessel. Between seal 61 a and seal 62a an annular outlet chamber communicates with the pressure vessel outletport 9 b. Between the seal 61 a and the seal 63 a an annular inletchamber communicates with the pressure vessel inlet port 9 a. Inlet port8 a is located to the left of seal 62 a and this seal ensures that inlet8 a is separated from outlet 9 b. Outlet 8 b is disposed to the right ofseal 63 a and this seal ensures that outlet 8 b is separated from inlet9 a.

The provision of the three annular seals in this way means that there isno need for any particular rotational alignment of the membrane moduleand the pressure vessel in embodiments such as that illustrated whereopenings 7 a, 7 b are provided at equal intervals around the entiremembrane module circumference.

The width of the annular inlet and outlet chambers is shown as W and thediameter of the inlet and outlet ports 9 a, 9 b is shown as D. The widthW is preferably greater than or equal to one quarter of the diameter D,in order to allow for the desired reduction in flow velocity downstreamof the inlet port 9 a and increase in flow velocity upstream of theoutlet port 9 b.

With larger values of W, the flow velocities into or out of the membranemodule are reduced, thereby reducing the likelihood of membrane damage.On the other hand, by minimising W, the diameter of the membrane module20 can be maximised for a given diameter of pressure vessel 4, and hencegreater use of available space can be achieved. These conflictingrequirements can be balanced if W is equal to one quarter of D, in whichcase the flow area of the port is equal to the cross-sectional area ofthe respective annular chamber. This produces good flow conditions,without excessive flow velocities which might damage the membranes,whilst making efficient use of the available space in the pressurevessel. More preferably, therefore, W is approximately equal to onequarter of D, for example within ±20% of one quarter of D. Whereadditional inlet and outlet ports 9 a, 9 b are used, as shown in dottedlines in FIG. 4, then the diameter D of the ports can be reduced and therequired width W of the annular chambers can be reduced by the sameratio.

The canister does not need to be designed for the same pressure as thepressure vessel, only to withstand the differential pressure equal tothe pressure drop between the tube and the shell side of the membranesat any point. A larger pressure variation, which may harm the membranesor the module, is prevented by the membrane protection system, 10,11,which is external of the pressure vessel. Additional reinforcement ofthe canister may however be necessary in the region of the seals forthese regions to achieve sufficient back pressure on the seals, so thatthe seals can function properly.

The canister is substantially rigid to facilitate its assembly into thepressure vessel. It may be made of metal or fibre reinforced plastics.In the latter case, the canister may be made by filament winding. Thewinding may be effected with rods in the mandrel to achieve the desiredopenings without reducing the mechanical strength of the canister.

The flange opening 5 allows for both easy installation and retraction ofthe canister as shown in FIG. 3. Retraction of the canister canpreferably be done with two textile straps glued into the potting orwound into the canister tube if composite material is selected. For moreslender canisters or a narrow annulus, where it might be morecomplicated to assemble the canister into the pressure vessel, both endsmay have a full diameter flange.

The seals in the annulus around the membrane module help to achieve thecorrect flow pattern. The seals at each end, 62 a and 63 a, of themodule prevent mixing of tube side fluid and shell side fluid in thevessel. The mid section seal 61 a prevents an undesirable shortcut ofthe shell side fluid. In both cases the pressure difference over theseals will not be higher than the pressure drop along the module plusany additional contribution from the surrounding process system. Theseseals should preferably be made of an inert material and according to adesign suitable for the purpose, and will in a preferred case be simpleO-rings or more compressible spring loaded lip seals. Where some axialmovement, due to thermal expansion are foreseen square profiles such asprovided by James Walker might be used. A “D” profile, which is stableagainst twisting, may be used.

The seal 61 a at the mid section of the annulus may be an inflatableseal type to ease the assembly. Such a seal can allow a wider gap andwill be inflated after assembly. Such seals are available from SealMaster Corporation or Mechanical Research & Design Inc. Inflatable sealsare of greatest use at ambient or low pressure applications, it notbeing generally practical to pressurise a seal to a pressure greaterthan natural gas pressure.

The membrane module should have a shell side feed arranged in such a waythat an even distribution of shell side fluid can be ensured so as toprotect the membranes from harmful inlet flow, such as high velocityflows which might cause cavitation. This can be prevented by adding areducer after the feed port, e.g. nozzle, or a bigger annulus in thissection of the pressure vessel. A preferred solution is to use a baffleinside the pressure vessel or to place the inlet opening(s) of thecanister on the opposite side of the pressure vessel inlet port.

The canister tube should preferably have multiple openings 7 a, 7 barranged to secure an even distribution of the flow into the membranesor it could have one elliptical slot. The optimised design will bedifferent for each particular case. The openings should be locatedaxially as close to the membrane potting as possible, in order to allowfor a counter current flow over a maximum axial length through themembrane module.

The tube side inlet and outlet of the membrane module are provided atthe potted ends thereof which communicate with the inlet and outletports 8 a, 8 b of the pressure vessel. Shell side inlet and outlet ports9 a, 9 b of the pressure vessel are arranged such that they communicatewith the corresponding openings 7 a, 7 b, when the module is installedin the pressure vessel. The shell side feed could also be throughmultiple nozzle connections, or through a ring chamber if furtherimproved distribution on the shell side is required.

The total area of the ports should be such that the fluid velocity onentrance into the membrane bundle, preferably does not exceed 500 mm/sfor a liquid and 5000 mm/s for a gas, or creates a too high pressuredrop.

There are many mass transfer processes in which the present inventionmay be applied, as is known in the art. The invention is particularlysuitable for removing carbon dioxide, hydrogen sulphide and water fromnatural gas, but this is just one possible use.

Although the use of a pressure vessel renders the apparatus suitable formass transfer processes at elevated pressures, the apparatus can also beused at ambient pressures. Typical elevated pressures at which theapparatus is useful are those in excess of 10 bar g (10⁶ N/m² aboveatmospheric pressure).

1. Membrane contactor apparatus comprising a bundle of axially extendingmembrane tubes arranged for a first fluid to flow outside of said tubesand for a second fluid to flow axially inside said tubes from an axialend of the bundle to an opposite axial end thereof, such that one ormore components of one of the fluids passes through walls of themembrane tubes into the other fluid, the apparatus further comprisinginlet means disposed axially inwardly of one of said axial ends of thebundle for introducing said first fluid into the bundle, wherein theinlet means comprises a plurality of circumferentially spaced inletopenings in the periphery of the bundle and is arranged to permit flowof said first fluid laterally into the bundle substantially around itsentire periphery, wherein the membrane tubes are arranged in a pluralityof layers extending laterally across the bundle, and wherein eachmembrane layer is exposed to at least one of said inlet openings. 2.Membrane contactor apparatus comprising a bundle of axially extendingmembrane tubes arranged for a first fluid to flow outside of said tubesand for a second fluid to flow axially inside said tubes from an axialend of the bundle to an opposite axial end thereof, such that one ormore components of one of the fluids passes through walls of themembrane tubes into the other fluid, the apparatus further comprisinginlet means disposed axially inwardly of one of said axial ends of thebundle for introducing said first fluid into the bundle, wherein theinlet means comprises a plurality of circumferentially spaced inletopenings in the periphery of the bundle and is arranged to permit flowof said first fluid laterally into the bundle substantially around itsentire periphery, and wherein at least two rows of circumferentiallyspaced inlet openings are provided in the periphery of the bundle, withthe centres of inlet openings in adjacent rows being circumferentiallyoffset.
 3. Apparatus as claimed in claim 1, wherein the bundle is housedin a housing having an inlet port for the first fluid, and wherein aninlet chamber is arranged to receive the first fluid from the inlet portand extends round the periphery of the bundle.
 4. Apparatus as claimedin claim 3, wherein the width of the inlet chamber, measured between aninside wall of the housing and the periphery of the bundle, is greaterthan or equal to one quarter of the width of the inlet port. 5.Apparatus as claimed in claim 1, wherein the first fluid is arranged toflow outside of said membrane tubes in the axial direction.
 6. Apparatusas claimed in claim 5, wherein the first and second fluid flows are inaxial directions opposite to each other.